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IR Channel
Introduction The WFC3 IR channel consists of a HgCdTe array Channel Specifications The 1024 x 1024 pixel HgCdTe array is sensitive to light on 1014 x 1014 pixels (18x18 μm pixels). It also contains a six stage thermoelectric cooler, which keeps the detector at 145K. The detector is divided into four quadrants of 512 x 512 pixels, each read independently from its outer corner pic. The detector is sensitive from 400-1700 nm, and optimized to wavelengths longer than ~1000 nm. Note that the IR filter set is limited to wavelengths above 800 nm, making it complementary to the UVIS channel. Each WFC3 R unit cell contains three transistors, which represent the versatility of the device, to an extent. IR Detector Basics Today's IR detectors are 2-D arrays of p-n (positive-negative) junctions working as photodetectors. In a p-n junction, negative charges jump from n-type material to p-type material, creating an electric field. This electric field creates a charged and highly resistive intermediate region with no free carriers. The strength of the electric field can be increased with a "negative bias" (an additional external electric field). An IR photon hits the photosensitive material, creating a free electron-hole pair. If the mobility of the charge carrier (the hole in the n-type material) is high enough, it will reach the depletion region before recombining. In the p-type region, it will recombine with an electron of an ionized hole, causing a voltage reduction. The voltage change can be measured, and is relative to the number of photons captured on each pixel. Compared to CCDs: IR detectors have higher read noise and dark current than CCDs, however there are many shortcomings of CCDs which are not seen in IR detectors. IR detectors allow the signal to be read out non-destructively (___What is non-destructive read out?___) multiple times, without affecting other pixels. This capability to sample the signal multiple times helps reduce read-out noise. IR detectors are immune to the charge bleeding (___What is charge bleeding?___) exhibited by CCDs at high signal levels. Saturation may still be a problem, though - pixels subject to the highest signal levels have higher dark-current rates ("image persistence") (___Read more on this and clarify.___). IR detectors do not show long-term on-orbit CTE degradation, a major benefit for later use of the detector's use. They are also capable of very high dynamic-range observations, due to multiple readout capability, and lack of charge bleeding. IR detectors can also recover pixels affected by cosmic rays noticed between adjacent reads. Problems and Solutions Problems Solved by WFC3: The NICMOS detectors (__what are the NICMOS detectors_?) have been affected by two problems - amplifier glow and bias jumps. Amplifier Glow (___What is amplifier glow?____) To eliminate amplifier glow completely, WFC3/IR uses external amplifiers rather than those directly on the MUX. (____what are amplfiiers ? Picture?___) Bias Jumps (___What are bias jumps?___) To reduce bias jumps, WFC3/IR uses reference pixels. Of the 1024 x 1024 pixels, the outer 5 columns and rows are insensitive to light. WFC3/IR is the first IR detector to use this strategy to correct for bias jumps. These reference pixels use fixed capacitances to provide constant-voltage reference values. There are two types of reference pixels - the pixels on the outer border of the detector, and the 4 inner rows and columns adjacent to the outer border. The border pixels are connected to capacitors located outside the unit-cells (___whats a unit cell?___), and are arranged in four sequentially increasing voltage levels. The inner four rows and columns of pixels are connected to on-board capacitors created within their cells, all of which provide the same reference signal. Reference pixels also track the low-frequency drift of the readout electronics and remove the "pedestal" variations that affected NICMOS. Pre-launch tests show reference-pixels are sensitive to detector temperature, and may help assess dark current expectatons. Expected Problems: There are many problems encountered with every detector, mainly those caused by noise and imperfections in the detector itself. There are also expected nuances caused by the telescope system (telescope breathing). Dark Current (___What is dark current?___) The WFC3/IR detector is refrigerated with a six-stage thermoelectric cooler to 145K. This temperature is unusually high for an IR detector, and required tailoring the composition of HgCdTe material to a wavelength cutoff of ~1700 nm. Despite the shoter wavelength cutoff, the complexity and limited lifetime of a stored-cryogen system is avoided (___what instruments use stored-cryogen systems?___). There is a high-band gap assiciated with the short cutoff wavelength, which also limits dark current and sensitivity to thermal background noise. Read Noise Each detector has four readout amplifiers (the four quadrants of the detector array, 512 x 512 pixels each), which all generate very similar amounts of read noise. (___Find more info on this.___) Flat Fields The WFC3/IR flat field images were generated using a pair of "lamp-off" and "lamp-on" exposures. The difference between the pair of exposures leaves the true flat-field response, and is a direct measure of QE uniformity. img Most of the non-uniformity seen in the flat field images is due to variations in composition of the HgCdTe material. Other defects are caused by blemishes on the detector surface. The circular patterned defects are caused by focusing errors in the optical path, and should be removed to better than 1% by using flat field images. Linearity and Saturation The IR detector response is slightly non-linear over the full range. pic Detector Cosmetics Hot pixels have more than 100 times the average dark current; that is, they have excessive charge compared to the surrounding pixels. Cold pixels are less sensitive to incident photons, and have a negative slope (__What is this slope?__). Cold pixels may be caused by WFC3/IR's lower intrinsic QE, or due to surface defects. Dead pixels do not respond at all to incoming light. Solution: The effects of bad pixels can be minimized by dithering observations.